This application is a non-provisional application claiming priority to provisional application Ser. No. 60/310,655, filed Aug. 6, 2001.
The demand for reduced size consumer electronics has produced a corresponding demand for reduced size electronic components used in these electronics. In portable electronics such as cellular telephones, one of the necessary components is an antenna. The most common type of antenna in cellular telephones are whip antennas because they are relatively cheap and simple to fabricate. However, the gain-bandwidth product of a whip antenna is relatively poor and the size is large.
Uniplanar compact photonic bandgap (UC-PBG) structures have been demonstrated in an attempt to reduce the size of antenna. One example of a UC-PBG structure 100 is shown in FIG. 1. This UC-PBG structure 100 contains a thin sheet of metal with a square lattice of Jerusalem crossed slots 112. The UC-PBG structure 100 may also be described as containing a unit cell 102 of a cloverleaf pattern with petals 104, a center 106, and a straight line segment 108 connecting adjacent centers 106. Neighboring cloverleaf patterns 102 are separated by a gap 110.
In another example of a UC-PBG, K. P. Ma et al. showed that if one places a PEC surface parallel to and electrically close to the UC-PBG structure, then the UC-PBG surface exhibits properties of a high-impedance surface, where it has a zero degree reflection phase for plane waves at normal incidence. Later, Richard Remski at Ansoft Corp. predicted that a conductor-backed UC-PBG also exhibited a full electromagnetic bandgap for surface waves. This was done using finite element (FEM) software to perform an eigenmode analysis on the open structure. However, only one publication addresses the question of a surface wave bandgap for the conductor-backed UC-PBG. No experimental data has yet been published.
While the UC-PBGs above were a step in the right direction, ample motivation still exists to develop antenna technology and apply it to practical filter and antenna applications at UHF and L-band frequencies (300 MHz to 2 GHz) for commercial wireless bands. Previous work on UC-PBGs, for example, demonstrated a fundamental parallel LC resonance near 10 GHz using a period of 0.12 inches, or λ/10 where λ is a free space wavelength at the fundamental resonance. However, for most practical applications the period must be reduced to be much less than λ/10, typically between λ/50 and λ/25. Accordingly, reduction of the unit cell dimensions is necessary.
Fries and Vahldieck disclosed an example of a patch antenna employing the simple UC-PBG in place of a metal patch. They demonstrate a 50% area reduction (0.707 reduction in linear dimensions) with no added manufacturing complexity when both the patch and ground plane have the UC-PGB feature. However, leakage of RF power through the slotted ground plane is a potential EMI concern. In this case, the fundamental resonant frequency of the UC-PBG unit cell was much higher than the fundamental resonant frequency of the patch antenna.
A DC inductive frequency selective surface (DCL FSS) can be used as a microstripline, resulting in a natural slow wave structure, which can be used to fabricate compact multi-band antennas. Some of these structures have been investigated, for example the use of printed periodic transmission lines, or simply an array of circular holes, as the ground plane of a microstripline. However, for many applications a solid ground plane must be used to control leakage and radiation into the rear hemisphere and to define the printed trace of the microstripline to be periodic. An example of this case is a one dimensional PBG cell proposed by Xue, Shum, and Chan. However, this is a 1-D patterned microstripline where only one layer of metal is used. The authors suggest an equivalent circuit, which includes a parallel LC network in series with the microstripline, however, the capacitance is quite small since it is defined by only edge-to-edge coupling. Hence, the fundamental resonant frequency is quite high, on the order of 5 GHz. This frequency is much too high for many conventional wireless applications operating at L band and below.